<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="./demo12.js"></script>
</head>
<body>
    <!---form表单的js操作--->
    <form action="">
        <input id="user_n" type="text" name="账号" value="请输入" placeholder="请输入用户名">
        <br>
        <input type="password" name="密码" value="123456">
    </form>
</body>
</html>